Three primary mechanisms move heat inside Earth: convection, conduction, and radiation. Each dominates in different layers, and together they slowly transport thermal energy from the planet’s deep interior to its surface, where Earth loses heat at a total rate of about 44 terawatts. Roughly 70% of that heat escapes through the ocean floor. To understand how this works, it helps to look at where Earth’s heat comes from and how each mechanism carries it outward.
Where Earth’s Heat Comes From
Earth’s internal heat has two main sources. The first is primordial heat, left over from the planet’s formation about 4.5 billion years ago. As smaller bodies collided and merged to build the early Earth, the kinetic energy of those impacts converted into thermal energy. Additional heat was generated when dense iron sank toward the center to form the core, releasing gravitational energy in the process. A significant fraction of that original heat remains trapped deep inside the planet today.
The second source is radioactive decay. Three long-lived elements, potassium, thorium, and uranium, slowly break apart and release energy as they do. Together, these elements account for roughly half of the heat Earth loses through its surface. They’re concentrated in the crust and upper mantle, which means the outermost layers of the planet are actively generating heat on their own, not just receiving it from below.
Conduction in the Crust and Lithosphere
Conduction is the simplest form of heat transfer: energy passes from one molecule to the next through direct contact, with no material actually moving. This is the dominant mechanism in Earth’s rigid outer shell, the lithosphere, which includes the crust and the uppermost mantle. Rock is a poor conductor compared to metals, so this process is slow. In the continental crust, the temperature rises by about 25°C for every kilometer of depth within the first 3 to 5 kilometers. That gradient doesn’t stay constant. By 40 kilometers deep, it drops to around 16°C per kilometer as the rock gets closer to the hotter, flowing mantle beneath.
Conduction matters most where rock is too rigid and cool to flow. In the oceanic lithosphere, for instance, heat moves vertically through the plate almost entirely by conduction. At the base of the plate, where temperatures are high enough for rock to start behaving like a very slow fluid, advection (heat carried by moving material) takes over. This transition zone between conductive and convective heat transport is a defining feature of plate structure.
Convection in the Mantle
Below the lithosphere, the mantle is hot enough and under enough pressure that solid rock flows over millions of years, behaving like an extremely viscous fluid. This is where convection becomes the dominant heat transport mechanism. Hot rock rises because it’s slightly less dense, spreads laterally, cools, and sinks again. This circulation pattern drives the movement of tectonic plates at the surface and is the primary way Earth sheds its internal heat.
The convection isn’t simple or uniform. Viscosity increases with depth, and the ability of rock to expand when heated (thermal expansivity) decreases. These changes with depth affect how vigorously different parts of the mantle circulate. Early in Earth’s history, when the interior was significantly hotter, convection may have operated in different regimes. Today’s system, called active-lid convection, features distinct plates that move and subduct. Hotter conditions could have produced sluggish-lid or stagnant-lid convection, where the surface layer was less mobile or essentially stationary.
One dramatic expression of mantle convection is the mantle plume: a column of unusually hot material rising from deep in the mantle, possibly from the core-mantle boundary itself. Plumes vary widely in their buoyancy and temperature. The excess heat they carry could come from conductive heating at the core-mantle boundary, from concentrations of radioactive elements in their source material, or from physical transfer of material between the outer core and the lower mantle. Research into chemical tracers in plume-fed volcanic rocks suggests that conductive heat transfer from the core, rather than direct mixing of core material into the mantle, is the more likely explanation for most plumes.
Radiative Heat Transfer in the Deep Mantle
At the extreme temperatures found in the lower mantle, a third mechanism becomes significant: radiative heat transfer. At these depths, rock is hot enough to emit and absorb infrared radiation, and this additional mode of heat transport changes how the deep mantle behaves. Radiative conductivity increases with temperature, meaning it plays a larger role the deeper you go.
This mechanism has important consequences for the large-scale structures in the lower mantle. Massive upwellings called superplumes, which can exceed 500 kilometers in width, appear to depend on radiative heat transfer for their stability. Without it, computer models predict extreme temperature contrasts of over 1,500°C between the interior of a plume and the surrounding mantle. Radiative transfer smooths out those contrasts, allowing broad, stable upwellings to persist rather than breaking apart into smaller, more chaotic features. The effect is especially pronounced near the core-mantle boundary, where temperatures exceed 3,500°C and radiation becomes an efficient way to redistribute thermal energy across short distances.
How These Mechanisms Work Together
The interior of the Earth isn’t governed by any single heat transport process. Instead, different mechanisms hand off to one another at different depths. At the core-mantle boundary, heat conducts (and radiates) from the liquid outer core into the lowermost mantle. That heat drives convective circulation through the mantle, carrying warm rock upward over tens of millions of years. When that rising material reaches the base of the lithosphere, convection gives way to conduction again, as heat passes slowly through the rigid plate to the surface.
The relative contribution of each mechanism depends on local conditions: temperature, pressure, rock composition, and whether the material is rigid or able to flow. In volcanically active regions or mid-ocean ridges, hot material reaches the surface directly, short-circuiting the slow conductive step. In old, thick continental crust, conduction dominates and surface heat flow is relatively low. The 44 terawatts Earth loses at its surface represents the combined output of all these processes, a planet still cooling from its violent formation while radioactive elements continue to stoke the fire from within.